Chandra/HETG observations of the recurrent nova RS Ophiuchi at day 13.9 of its 2006 outburst reveal a spectrum covering a large range in plasma temperature and characterized by asymmetric and blue-shifted emission lines. We investigate the origin of
these asymmetries and broadening of emission lines. We perform 3-D hydrodynamic simulations of the blast wave from the 2006 outburst, propagating through the inhomogeneous CSM. The model takes into account the thermal conduction (including the effects of heat flux saturation) and the radiative cooling. From the simulations, we synthesize the X-ray emission and derive the spectra as they would be observed with Chandra/HETG. Our model reproduces the observed X-ray emission in a natural way if the CSM in which the outburst occurred is characterized by an equatorial density enhancement. Our ``best-fit model predicts that most of the early X-ray emission originates from a small region propagating in the direction perpendicular to the line-of-sight and localized just behind the interaction front between the blast wave and the equatorial density enhancement. The model predicts asymmetric and blue-shifted line profiles remarkably similar to those observed. These asymmetries are due to substantial X-ray absorption of red-shifted emission by ejecta material. The comparison of high quality data of Chandra/HETG with detailed hydrodynamic modeling has allowed us to unveil, for the first time, the details of the structure emitting in the X-ray band in early phases of the outburst evolution, contributing to a better understanding of the physics of interactions between nova blasts and CSM in recurrent novae. This may have implications for whether or not RS Ophiuchi is a Type Ia SN progenitor system.
The Balmer line profiles of nonradiative supernova remnant shocks provide the means to measure the post-shock proton velocity distribution. While most analyses assume a Maxwellian velocity distribution, this is unlikely to be correct. In particular,
neutral atoms that pass through the shock and become ionized downstream form a nonthermal distribution similar to that of pickup ions in the solar wind. We predict the H alpha line profiles from the combination of pickup protons and the ordinary shocked protons, and we consider the extent to which this distribution could affect the shock parameters derived from H alpha profiles. The Maxwellian assumption could lead to an underestimate of shock speed by up to about 15%. The isotropization of the pickup ion population generates wave energy, and we find that for the most favorable parameters this energy could significantly heat the thermal particles. Sufficiently accurate profiles could constrain the strength and direction of the magnetic field in the shocked plasma, and we discuss the distortions from a Gaussian profile to be expected in Tychos supernova remnant.
